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Formula for Calculating BTU Ratings for Heat Exchangers

Ronald Draper on Thu November 17, 2011 2:26 AM User is offline

This is a general question.

I have seem ratings for aftermarket BTU ratings.

I want to verify their numbers but I have not found a source on the net to do so.

I did find in a mid fifties book the following basic formula.

Using the number of fins per inch times two for both sides of the fin less the area occupied by the holes for the tubing times the width and height and depth of the heat exchanger.

Can some one provide a link to a formula?



HECAT on Thu November 17, 2011 6:55 AM User is offline

you may find the formula you seek here


HECAT: You support the Forum when you consider for your a/c parts.


ice-n-tropics on Thu November 17, 2011 7:23 PM User is offline

It's deer season in Texas but you have a great question.
The theorists in automotive A/C have used the book "Compact Heat Exchangers" as their bible. It was confirmed in association with Harrison Radiator Division of GM. My copy focuses on the Nesselt Number Method.
Reputable heat exchanger manufacturers test their coils under ASHRAE methods and publish results. These graphs are usable, but, do not take into account the heat transfer loss due to oil circulating with refrigerant. A 95% effectiveness factor due to oil circulation is recommended for mobile A/C systems design.
Down in the hollar
Sittin on a log
Finger on the trigger
Eye on the hog

Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy. How To Air Condition Your Hot Rod

Edited: Thu November 17, 2011 at 7:26 PM by ice-n-tropics

NickD on Fri November 18, 2011 4:57 AM User is offline

In the real world, BTU output varies considerably with the type of refrigerant used, ambient temperature, humidity, and the type of system the evaporator is employed in. As a worse case example, in subzero temperatures, the refrigerant remains a liquid, doesn't evaporate, and effectively is zero BTU output. So at best these numbers are a figure of merit to compare one evaporator against another under identical conditions.

But various configurations will produce different performance curves over the refrigerant, temperature, humidity range, typically not specified.

Also in the real automotive world, the stylist hold the key power, marketing figures if the vehicle won't sell, no use in producing it. Rest of us had to work in the space provided. Still consider thermodynamics more of an art than a science. But have fun working with those equations. Yet another key factor in a design, how cheaply can it be made? One of the key rulings in automotive in particular. Materials used are generally determined by a materials engineer, but mostly by the purchasing department. But key engineering takes the blame if the warranty rates get excessive.

University courses just teach this theory with practically zero on the real world aspects an engineer has to deal with. So your exercise will be theory at best.

Ronald Draper on Fri November 18, 2011 11:10 PM User is offline

While deer season is seasonal in Texas - it is always a/c season in Dallas.

I am always wondering about numbers. I understand that there is a degree of variation between theory and reality. And how companies will spend a dime to save a nickel - America lives on warranties.....

Using a modern example if I may - govmo used the A6 compressor for a great number of years with great success. Then the bean counters took over with a choke hold. Look at the money they spent to come up with new compressors (what is it now - five or six generations and still planning another?) that barely make it through warranty. Then the consumer is forced to spend any money they may have saved through better gas millage to get it repaired. How is that helping people?

Enuff of the crying in the beer....

The link is to a tube and shell article - I will look it over and compare to the tube and fin fifties book.

Is there a simple "calculator" net program some where that takes the basic info and does the math for you?

Thanks for your inputs.


NickD on Sat November 19, 2011 7:26 AM User is offline

Had an AC engineer on this board for awhile, DetroitAC I believe his handle was, used an AC CAD program, but never learned the name of it. Gather programs of this type cost a fortune. I was using a simulator electronic program that was in the $20,000 price range. Guess the reason why DetroitAC is no longer here is that most of our jobs were moved to China. Like mine, been keeping contact with my fellow engineers, but that quickly dies off. Some have gone into other fields like home construction or real estate, but that also died. Making a few bucks going way back to my FAA and FCC certification to help pay my outrageous property tax bill, but also many budget cuts in this area. Hell, I am 72 years of age already. Can't say I really miss that pressure of time to market.

Disturbed that over 50 years had to pay income tax on my forced FICA deductions, then have to pay income tax on 85% of my measly SS benefits. But by working still come out a few bucks ahead. No longer interested in improving performance in AC systems, if it didn't work well from the factory, either live with it or get rid of the vehicle. Just keep them stock and working as well as the rest of the vehicle. A key performance detriment is debris in the system, really not an AC mechanic, but a janitor.

In 1965 a 20" color TV was 700 bucks, a new vehicle was four times that amount. Today, a much improved TV that size runs about 125 bucks and these new vehicles should also be at the same ratio. All that chrome, steel, cast iron, bumper jacks have been replaces with plastic and aluminum, the latter requires the minimum amount of machining. Cars today should be no more expensive than they were in 1965 or even cheaper even with inflation. Can tell you, we had to deal with fraction of a cent savings. American public is really getting a major ripoff. Your government is the major cause of this.

HECAT on Sat November 19, 2011 11:38 AM User is offline

Originally posted by: Ronald Draper
The link is to a tube and shell article - I will look it over and compare to the tube and fin fifties book. Is there a simple "calculator" net program some where that takes the basic info and does the math for you?

The link was meant to provide information about the many heat exchange formulas and calculation theory that is used. I don't know what formula may or may not apply to your research, but Ice-n-tropics may have eluded to looking closer at the Nesselt Number Method (IDK). It was also meant to offer some thought about it just not being that "simple".


HECAT: You support the Forum when you consider for your a/c parts.


bohica2xo on Sun November 20, 2011 3:29 AM User is offline

Calculator? Sure, it is already on many computers. Open Excel.

Q = U A ?Tlm

H = 4.5 q (h2 - h1) ?


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Ronald Draper on Mon November 21, 2011 9:30 PM User is offline

I understand that computer programs make things simpler and math is not my strong suit.

I am not trying to squeeze more out of existing units but if I were to purchase something - I want to know what I am buying. It is an apples to apples comparison rather than an apple to oranges comparison.

I have a couple of evaporators that I would like to get the BTU ratings on.

One is from a 1955 Lincoln trunk mounted factory air system and it is huge.

The size in inches is 6 by 17 5/8 by 9 with eleven fins to the inch with 18 pipes of 3/8 spread across three circuits.

The second is from a late sixties under dash unit.

The size in inches is 2 3/4 by 13 3/4 by 4 with thirteen fins to the inch with 10 pipes of 3/8 spread across a single circuit.

Not being able to see the insides of a modern under dash evaporator makes it difficult to know their dimensions but I would think that most under dash units are close and they rate theirs at 18,500 BTU.

My math does not come close on the little coil I have so it would be hard to say what the Lincoln was rated at

Ignoring the holes that pipes run through, the areas are 20938.5 and 3932.5 respectively and includes doubling for the two sides of the fins. That would further multiplied by three for forced convection (62815.5 and 11797.5).

The Lincoln could not have had a 6 ton rating with a 10 cu in compressor even at a 100 mph.

Chrysler rated their systems of the late fifties at 2.5 tons - govmo could have been close to 4 tons at 100 mph with the 13 cu in compressors.

These are R-12 based numbers (I guess) but my old book does not talk about the refrigerant. So I am wondering what I have missed?



PS - is there a means of posting an embedded picture?

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